Si Nanopillar Arrays Research Articles

Design of mid-wave infrared achromatic double-layer metalens with enhanced efficiency

Achromatic metalenses in the mid-wave infrared (3–5 μm), known for their light weight, CMOS compatibility, and ultra-compactness, offer significant potential in astronomy, security inspections, and health security. However, previous endeavors have been hindered by underdeveloped material technology and relatively low efficiency. To address these challenges, this study introduces an enhanced-efficiency mid-wave infrared achromatic double-layer metalens, featuring a top-layer ZnS nanopillar array and a bottom-layer Si nanopillar array on a Si substrate. Utilizing this approach, we numerically demonstrate both polarization-insensitive and polarization-controlled varifocal broadband achromatic metalenses. For the polarization-insensitive metalens, the double-layer design provides achromatic focusing comparable to the all-Si counterpart, with a focal length of 133 µm, a focal length shift within ± 5.5%, and Strehl ratios above 0.8. However, the average focal efficiency improves from 40.8% (all-Si) to 50.2% (double-layer). Additionally, both all-Si and double-layer polarization-controlled varifocal achromatic metalenses show similar focusing abilities, with focal lengths of about 137 and 173 µm under X and Y linearly polarized light, respectively. Yet, the double-layer varifocal metalens achieves focal efficiencies of 46.4% and 52.7%, an improvement of 13.1% and 17.6% under X and Y linearly polarized light, respectively.

Broadband anti-reflection coating for Si solar cell applications based on periodic Si nanopillar dimer arrays & Si3N4 layer

A structure of periodic Si nanopillar dimer array & Si3N4 layer which sits on Si substrates is presented to obtain a broadband high transmission and low reflection. We show numerically that the average reflection of this structure can reach 1.8%, and the average transmission can reach 93.1% in the 400–1100 nm range, due to the combined effects of the forward scattering effects of Si nanopillar dimers and Si3N4 layer’s anti-reflection effects. Si nanopillars’ diameter and height, Si3N4 layer’s height, the gap of dimers, and the period of the array have significant impacts on the transmittance and reflection. This work supplies a practicable way for decreasing broadband surface reflection and increasing the absorption of light for Si solar cell applications.

Tunable Periodic Nanopillar Array for MAPbI3 Perovskite Photodetectors with Improved Light Absorption.

In the field of optoelectronic applications, the vigorous development of organic-inorganic hybrid perovskite materials, such as methylammonium lead triiodide (MAPbI3), has spurred continuous research on methods to enhance the photodetection performance. Periodic nanoarrays can effectively improve the light absorption of perovskite thin films. However, there are still challenges in fabricating tunable periodic patterned and large-area perovskite nanoarrays. In this study, we present a cost-effective and facile approach utilizing nanosphere lithography and dry etching techniques to create a large-area Si nanopillar array, which is employed for patterning MAPbI3 thin films. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) results reveal that the introduction of nanopillar structures did not have a significant adverse effect on the crystallinity of the MAPbI3 thin film. Light absorption tests and optical simulations indicate that the nanopillar array enhances the light intensity within the perovskite films, leading to photodetectors with a responsivity of 11.2 A/W and a detectivity of 7.3 × 1010 Jones at 450 nm in wavelength. Compared with photodetectors without nanostructures, these photodetectors exhibit better visible light absorption. Finally, we demonstrate the application of these photodetector arrays in a prototype image sensor.

Hydrophobic Si nanopillar array coated with few-layer MoS2 films for surface-enhanced Raman scattering

In this study, we covered Si nanopillar (NP) array with few-layer MoS2 films to convert their wettability characteristics from hydrophilic to hydrophobic for applications as a surface-enhanced Raman scattering (SERS) substrate. The Si NP array was fabricated using a semiconductor process. We then sulfurized and transferred MoO3 films coated onto the Si NP array to MoS2 films. The surface morphology and cross-sectional profile of the MoS2-coated Si NP array structure was examined using scanning electron microscopy and transmission electron microscopy. The SERS results indicate that the substrate exhibits a favorable enhancement factor of 1.76 × 103 and a detection limit of approximately 10−5M for Rhodamine 6G (R6G) utilized as the test molecule, attributed to the charge transfer (CT) mechanism at the interface between MoS2 and R6G. Contact angle measurements showed that the MoS2-coated Si NP array possesses a hydrophobic surface. Our results suggest that an MoS2-coated Si NP array with CT and hydrophobicity characteristics is extremely promising SERS substrates for SERS applications.

Roll-to-roll reactive ion etching of large-area nanostructure arrays in Si: Process development, characterization, and optimization

Roll-to-roll (R2R) nanofabrication processes are recognized as key enabling-technologies for many next-generation applications in flexible electronics, displays, energy generation, storage, as well as healthcare. However, R2R processing techniques reported in the literature currently lack a scalable method of performing high-throughput nanoscale pattern transfer of geometry requiring a high degree of fidelity in terms of critical dimension resolution, etch uniformity, and aspect ratio. Reactive ion etching (RIE) addresses the need for sub-10 nm pattern transfer with large-area uniformity in wafer-scale semiconductor manufacturing, but adapting plasma etch systems for use in R2R nanopatterning has proven to be nontrivial. Moreover, robust models for simulating R2R RIE do not exist, which is an obstacle to the creation of computational approaches to design, control, and scale-up of nanoscale R2R equipment and processes. To address these challenges, we demonstrate a process flow for fabricating Si nanopillar arrays utilizing a combination of nanoimprint lithography and RIE with all pattern transfer steps performed using a R2R plasma reactor system. Specifically discussed are process development details for etching imprint resist and Si including etch rates, cross-web etch uniformity, etch directionality, and etch selectivity at varying gas chemistries, powers, and pressures. 2k full-factorial Design of Experiments (DoEs) and ordinary least-squares regression analysis are also employed to study influence of process parameters on multiple outgoing etch quality characteristics and generate stochastic models of the R2R RIE pattern transfer process into Si. Utilizing these DOE-based models and desired targets for etch quality characteristics, we describe a bounded multivariate inverse-optimization scheme for automated etch process parameter tuning. The culmination of these efforts, to the best of the authors' knowledge, is the first reported RIE-based pattern transfer of 100 nm-scale features performed in continuous R2R fashion with control of feature geometry over large area. The methodology employed herein may be applied similarly to additional materials and geometries for future applications.

Fabrication of ultrahigh aspect ratio Si nanopillar and nanocone arrays

High aspect ratio (HAR) structures have many promising applications such as biomedical detection, optical spectroscopy, and material characterization. Bottom-up self-assembly is a low-cost method to fabricate HAR structures, but it remains challenging to control the structure dimension, shape, density, and location. In this paper, an optimized top-down method using a combination of pseudo-Bosch etching and wet isotropic thinning/sharpening is presented to fabricate HAR silicon (Si) nanopillar and nanocone arrays. To achieve these structure profiles, electron beam lithography and reactive ion etching were carried out to fabricate silicon pillars having a nearly vertical sidewall, followed by thinning or sharpening by wet etching with a mixture of hydrofluoric (HF) acid and nitric acid (HNO3). For the dry etching step using the pseudo-Bosch process, the sidewall angle is largely dependent on the SF6/C4F8 gas flow ratio, and it was found that a vertical profile can be attained with a ratio of 22/38. For the wet etching process, a very large HNO3/HF volume ratio is shown to give smooth etching with a slow and controllable etching rate. The final structure profile also depends on the pattern density/array periodicity. When the array period is large, silicon nanopillar is thinned down, and its aspect ratio can reach 1:135 with a sub-100 nm apex. When the pillar array becomes very dense (periodicity much smaller than height), a very sharp nanocone structure is obtained after wet etching with an apex diameter under 20 nm.

High aspect ratio arrays of Si nano-pillars using displacement Talbot lithography and gas-MacEtch

Structuring Si in arrays of vertical high aspect ratio pillars, ranging from nanoscale to macroscale feature dimensions, is essential for producing functional interfaces for many applications. Arrays of silicon 3D nanostructures are needed to realize photonic and phononic crystals, waveguides, metalenses, X-ray wavefront sensors, detectors. In particular, arrays of Si nanopillars are used as bio-interfaces in neural activity recording, cell culture, microfluidics, sensing and on-chip manipulation. Here, we demonstrate a strategy for realizing arrays of protruding sharp Si nanopillars, using displacement Talbot lithography combined with metal-assisted chemical etching (MacEtch) in gas phase. Such combination enables reliable and low cost pathway for fabrication of ordered nanopillars arrays on large scale. With the double exposure of a linear grating mask in orthogonal orientations and the lift-off technique, we realized a catalyst pattern of holes in a Pt thin film with a period of 1 μm and hole diameter in the range of 100–250 nm. MacEtch in gas phase by using vapor HF and oxygen from air allows to etch arrays of protruding Si nanopillars 200 nm-thick and aspect ratio in the range of 200 (pillar height/width) with an etching rate up to 1 μm/min. Gas-MacEtch has the advantage of no capillary stiction, no ion beam damage of the Si substrate, nanometric resolution and high fidelity of pattern transfer. In combination with controlled uniformity of feature size on large area, spatial frequency doubling and high resolution of Talbot lithography, the proposed method is an easy-to-scale-up processing that can support the fabrication of Si pillars arrays for many valuable applications both at micro and nano-scale.

Manipulation of optical bound states in the continuum in a metal-dielectric hybrid nanostructure

Optical bound states in the continuum (BICs) are spatially localized states with vanishing radiation, despite their energy embedded in the continuum spectrum of the environment. They are expected to greatly enhance light–matter interaction due to their long lifetime and high quality factor. However, the BICs in all-dielectric structures generally exhibit large mode volumes and their properties are difficult to manipulate. In this paper, we propose a metal–dielectric hybrid nanostructure where a silver film is inserted into the silicon (Si) substrate under the Si nanopillar array. We show that symmetry-protected BIC in this system can couple with surface plasmon polaritons (SPPs) to form a hybridized mode. Compared with previous symmetry-protected BICs in all-dielectric structures, the SPP-coupled BIC has a significantly decreased mode volume, and its corresponding electric field is strongly localized below the Si nanopillars. We also show that the SPP mode makes the original polarization-independent symmetry-protected BIC become polarization-dependent. In addition, we demonstrate that the silver film in the considered structure can induce a metal mirror effect. The destructive interference between the magnetic dipole inside the Si nanopillars and the mirror magnetic dipole in the silver film can lead to the formation of accidental BICs. Our hybrid structure provides a versatile platform for the manipulation of light–matter interaction in the nanoscale.

Surface lattice resonance based on periodic arrays of Si nanopillar dimers

The periodic array of Si nanopillar dimers is proposed to form surface lattice resonance (SLR) with strong local fields and narrow line-width. In the array, Si nanopillar dimers sit on quartz substrates. The simulated results show that the line-width of SLR can be as small as 4.8 nm. While SLR can't be generated by the periodic Si nanopillar array. The advantage of introducing Si nanopillar dimers is that the near field coupling between Si nanopillar dimers enhances the Mie polarization of Si nanopillar dimers. It is conducive to generating SLR, which is originated from the coupling between the Mie polarization of individual Si nanopillar dimers and diffracted waves. The gap between dimers, Si nanopillar dimers' diameter and height, the arrays' period and the refractive index of the medium environment have important influences on SLR. This work offers a possible way for SLR applications.

High Aspect Ratio Arrays of Si Nano-Pillars Using Displacement Talbot Lithography and Gas-Macetch

High Aspect Ratio Arrays of Si Nano-Pillars Using Displacement Talbot Lithography and Gas-Macetch

Plasmon lattice resonances induced by an all-dielectric periodic array of Si nanopillars on SiO2 nanopillars

An all-dielectric periodic array is proposed to form plasmon lattice resonances (PLR). In the array, Si nanopillars are on top of SiO2 nanopillars, and SiO2 nanopillars are on top of quartz substrates. The simulated results show that the line-width of the PLR can be as small as 3.3 nm. This can be attributed to the coupling between the Mie resonances of Si nanopillars and the diffracted waves. While the PLR can’t be formed by the periodic Si nanopillar array directly sitting on quartz substrates. The diameter and height of Si nanopillars, the period of the array and the height of SiO2 nanopillars have significant impacts on the PLR. This work extends the application of PLR.

Si nanopillar arrays as possible electronic device platforms

This paper explores the possible use of silicon nanopillars as electronic devices. The silicon nanopillars studied in this work were fabricated by electron beam lithography and by plasma ion etching of silicon wafers. The electrical and physical properties of these pillars with full width at half maximum ranging from few nanometers to 100 nm and length of few hundreds of nanometer have been investigated by time-resolwed photoluminescence, infrared spectroscopy and current-voltage characteristics. An ultrafast blue luminescence component competing with non-radiative recombination at surface defects was quantified as originating from the no-phonon recombination. This component involved two decay processes with a peak energy at around 500 nm, which have the fast component close to femtosecond time scale. The emission exhibits also a slow component in the red spectral region with a time constant in the nanosecond regime. The nanopillars have been smoothened in an attempt to passivate surface defects. The results are indicative of an increase in the lifetimes-of carriers and an enhancement in the red component of the emission with much slower sates. The presence of ultrafast decay at blue-green spectral region is suggestive of the possibility of using the silicon nanopillars as ultrafast switching devices. Silicon nanopillar arrays can also be an ideal platform in trapping and sensing chemical or biological species using diamond NV centers.

Using Si/MoS2 Core-Shell Nanopillar Arrays Enhances SERS Signal.

Two-dimensional layered material Molybdenum disulfide (MoS2) exhibits a flat surface without dangling bonds and is expected to be a suitable surface-enhanced Raman scattering (SERS) substrate for the detection of organic molecules. However, further fabrication of nanostructures for enhancement of SERS is necessary because of the low detection efficiency of MoS2. In this paper, period-distribution Si/MoS2 core/shell nanopillar (NP) arrays were fabricated for SERS. The MoS2 thin films were formed on the surface of Si NPs by sulfurizing the MoO3 thin films coated on the Si NP arrays. Scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were performed to characterize Si/MoS2 core-shell nanostructure. In comparison with a bare Si substrate and MoS2 thin film, the use of Si/MoS2 core-shell NP arrays as SERS substrates enhances the intensity of each SERS signal peak for Rhodamine 6G (R6G) molecules, and especially exhibits about 75-fold and 7-fold enhancements in the 1361 cm−1 peak signal, respectively. We suggest that the Si/MoS2 core-shell NP arrays with larger area could absorb more R6G molecules and provide larger interfaces between MoS2 and R6G molecules, leading to higher opportunity of charge transfer process and exciton transitions. Therefore, the Si/MoS2 core/shell NP arrays could effectively enhance SERS signal and serve as excellent SERS substrates in biomedical detection.

Spectral Imaging and Computer Vision for High-Throughput Defect Detection and Root-Cause Analysis of Silicon Nanopillar Arrays

Abstract Far-field spectral imaging, coupled with computer vision methods, is demonstrated as an effective inspection method for detection, classification, and root-cause analysis of manufacturing defects in large area Si nanopillar arrays. Si nanopillar arrays exhibit a variety of nanophotonic effects, causing them to produce colors and spectral signatures which are highly sensitive to defects, on both the macro- and nanoscales, which can be detected in far-field imaging. Compared with traditional nanometrology approaches like scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical scatterometry, spectral imaging offers much higher throughput due to its large field of view (FOV), micrometer-scale imaging resolution, sensitivity to nm-scale feature geometric variations, and ability to be performed in-line and nondestructively. Thus, spectral imaging is an excellent choice for high-speed defect detection/classification in Si nanopillar arrays and potentially other types of large-area nanostructure arrays (LNAs) fabricated on Si wafers, glass sheets, and roll-to-roll webs. The origins of different types of nano-imprint patterning defects—including particle voids, etch delay, and nonfilling—and the unique ways in which they manifest as optical changes in the completed nanostructure arrays are discussed. With this understanding in mind, computer vision methods are applied to spectral image data to detect and classify various defects in a sample containing wine glass-shaped Si resonator arrays.

Enhancing the device performance of SiNP array/PtTe2 heterojunction photodetector by the light trapping effect

In this study, we have demonstrated the fabrication of a highly sensitive photodetector, which was based on Si nano-pillar (SiNP) array/PtTe2 heterojunction that can work under a self-powered mode of operation. The as-fabricated heterojunction exhibits the obvious current rectifying behavior with a rectification ratio of 6 × 103 at a bias voltage of ±1 V. Moreover, it displayed an intriguing photoelectric ability to a wide range of illumination from ultraviolet to near-infrared light. Under light illumination at 980 nm, the photodetector exhibits a responsivity of 0.71 A W−1, a specific detectivity of 2.81 × 1011 Jones, and an external quantum efficiency of 89.9 %, respectively. More importantly, the device has a fast response speed of τraise/τfall of 6.21 / 26.3 μs. Thanks to the light trapping effect of the surficial nanostructure, the SiNP array effectively improves the absorption of the heterojunction, and therefore enhances the detecting performance of the device. Moreover, the device is able to record a near-infrared letter with acceptable resolution. All of these results suggest that present two-dimensional material may have potential application in future high-performance optoelectronic devices.

Ordered arrays of Si nanopillars with alternating diameters fabricated by nanosphere lithography and metal-assisted chemical etching

Ordered Si nanopillar arrays have a great potential for e.g. photonic, sensing and electronic devices. In the present article, we investigate the tailoring of Si nanostructure geometries obtained on large areas by nanosphere lithography combined with metal-assisted wet-chemical etching. In particular, the formation of Si nanopencils and, as a new feature of metal-assisted chemical etching, of nanopillars with single and multiple constrictions is demonstrated, which is accomplished by varying the HF/H2O2 ratio of the etching solution without modifying the noble metal film. As a result, it is possible to obtain defined constrictions in high aspect ratio Si nanopillars (length-to-diameter up to 27) with relative diameter reductions of up to around 0.6. Moreover, by controlled oxidation in water vapor nanoscale Si inclusions surrounded by an amorphous SiO2 shell are obtained. The morphology and structure of these pillar arrays are analyzed by scanning and transmission electron microscopy, complemented by optical reflection measurements. For the pencil-shaped Si nanostructures a reflectivity lower than 3% is found in the visible and infrared wavelength ranges. Nanopillars with multiple constrictions exhibit a significantly reduced thermal conductivity due to phonon backscattering. Therefore, the presented method could be implemented in the large-scale fabrication of advanced thermoelectric devices.

Surface wettability of silicon nanopillar array structures fabricated by biotemplate ultimate top-down processes

We fabricated high aspect ratio 10-nm Si nanopillar (NP) array structures with a few-tenths-nm-gap arranged by fusing biotemplate and neutral beam etching processes to investigate the wettability [e.g., contact angle (CA)] with and without surface silicon oxide film. The NP array with silicon native oxides in all gaps exhibited super-hydrophilicity due to the chemical liquid-solid interface interaction and larger surface area than the Si flat surface thanks to the NP structure. These phenomena can be explained by using the Wenzel model. In contrast, when we selectively removed the native oxide on Si NP surface with our radical treatment, a gap variation from 11 to 43 nm stably resulted in a CA of more than 96° (hydrophobicity) with a maximum of 115°. This can be explained by using the Cassie–Baxter model with a filling factor. Our findings demonstrate that controlled surface wettability can be achieved by combining our controllable gap silicon NP array structures and the surface with or without silicon native oxides. The gap of a Si NP fills with water due to the capillarity on a silicon native oxide, but on a pure stable silicon one with a defect-free surface, does not completely fill. We found that Si NP structures with controllable gaps exhibit a surface wettability ranging from super-hydrophilicity to high-hydrophobicity.

Omnidirectional and Broadband Antireflection Effect with Tapered Silicon Nanostructures Fabricated with Low-Cost and Large-Area Capable Nanosphere Lithography.

In this report, we present a process for the fabrication and tapering of a silicon (Si) nanopillar (NP) array on a large Si surface area wafer (2-inch diameter) to provide enhanced light harvesting for Si solar cell application. From our N,N-dimethyl-formamide (DMF) solvent-controlled spin-coating method, silica nanosphere (SNS in 310 nm diameter) coating on the Si surface was demonstrated successfully with improved monolayer coverage (>95%) and uniformity. After combining this method with a reactive ion etching (RIE) technique, a high-density Si NP array was produced, and we revealed that controlled tapering of Si NPs could be achieved after introducing a two-step RIE process using (1) CHF3/Ar gases for SNS selective etching over Si and (2) Cl2 gas for Si vertical etching. From our experimental and computational study, we show that an effectively tapered Si NP (i.e., an Si nanotip (NT)) structure could offer a highly effective omnidirectional and broadband antireflection effect for high-efficiency Si solar cell application.

All‐Dielectric Nanostructure Fabry–Pérot‐Enhanced Mie Resonances Coupled with Photogain Modulation toward Ultrasensitive In2S3 Photodetector

AbstractAs the fresh blood of 2D family, non‐layered 2D materials (2DNLMs) have demonstrated great potential in the application of next‐generation optoelectronic devices. However, stemming from the weak light absorption brought by atomically thin thickness and the interfacial recombination brought by surface dangling bonds, traditional 2DNLM photodetectors are always accompanied by limited performance. Herein, a structure that integrates Si nanopillar array and non‐layered 2D In2S3 to construct an ultrasensitive photodetector is designed. In particular, periodically Si nanopillars can act as Fabry–Pérot‐enhanced Mie resonators that can effectively control and enhance the light absorption of 2D In2S3. On the other hand, a vertical built‐in electric field is introduced in the In2S3 channel to capture photogenerated holes and leave electrons recycling in In2S3, obtaining a high photogain. Benefiting from these two mechanisms, this proposed photodetector presents a high responsivity of 4812 A W−1 and short rise/decay time of 5.2/4.0 ms at the wavelength of 405 nm. Especially, a high light on–off ratio greater than 106 and a record‐high detectivity of 5.4 × 1015 Jones are achieved, representing one of the most sensitive photodetectors based on 2D materials. This deliberate device design concept suggests an effective scheme to construct high‐performance 2DNLM optoelectronic devices.

(Invited) Study of Mechanical Behaviour of Si Anode upon Electrochemical Lithium Insertion

A battery is one of the representative electrochemical systems studied widely for energy storage applications. In particular, Li-ion batteries are becoming an essential component in mobile electronic devices and electric vehicles, as smaller, lighter, and longer energy storage is required. Silicon is considered as one of the most promising anode materials for Li-ion batteries because of its exceptional specific capacity, which is about ten times that of commercial graphite anodes. However, conventional Si anodes typically suffer from rapid capacity decay due to mechanical fracture caused by large volume changes (300%) during repeated electrochemical lithium insertion and extraction. It this talk, I will present the mechanical behaviour of various structure of single crystalline Si nanopillar upon electrochemical lithiation. Single crystalline Si nanopillar array fabricated by top-down process provides well defined dimension and crystalline orientation. The study of lithiated Si nanopillar exhibited interesting phenomena such as anisotropic expansion, size dependency of mechanical fracture, and temperature dependency [1-4]. Single crystalline Si plate various bending behaviour for the crystalline orientation and its large fracture resistance [5]. These studies will provide better understanding of mechanical behaviour of lithited Si anode for design of robust Si anode electrode. Acknowledgement The contents in this paper were published in references [1-5].